1. Field of the Invention
The present invention relates to a propulsive force controlling apparatus which is applicable to a marine vessel having a propulsion system, a marine vessel maneuvering supporting system and a marine vessel each including the propulsive force controlling apparatus, and a propulsive force controlling method.
2. Description of the Related Art
A propulsion system including an air-assist type engine and a multi-stage gear is conventionally used for causing a small-scale marine vessel (e.g., a boat) to run at a very low speed. The very low speed operation is required when a trolling operation is performed or the marine vessel is moved toward or away from a wharf.
However, the aforementioned propulsion system is not popular because of its complicated structure and high costs.
On the other hand, a hydraulic clutch controlling technique for a marine vessel engine is disclosed in Japanese Examined Patent Publication No. 06-68292 (1994). With the hydraulic clutch controlling technique, a multi-disk clutch is controlled to be switched alternately between a half-coupling state and a direct-coupling state to provide a desired trolling speed.
However, slippage occurs between clutch disks in the half-coupling state, thereby making it difficult to accurately control a propulsive force by detecting the rotational speed of a drive shaft provided at a stage before the clutch. With the hydraulic clutch controlling technique disclosed in Japanese Examined Patent Publication No. 06-68292, detection and feedback of the rotational speed of a propeller shaft provided at a stage subsequent to the clutch is required for accurate control of the propulsive force.
An outboard motor (another type of propulsion system) conventionally includes a dog clutch. The dog clutch does not have the half-coupling state, but has a coupling state and a decoupling state. Therefore, it is necessary to reduce an engine speed for reducing the trolling speed. However, it is impossible to reduce the engine speed to lower than an idling speed, making it impossible to perform the trolling operation at a very low speed.